The invention involves a device for melting or refining glass or glass ceramics.
Devices of this type have become known in the form of so-called skull crucibles. They contain a crucible wall 10. This wall 10 is generally cylindrical. It is made out of a ring of vertical metal pipes 12. Slits 14 remain between pipes 12 that are adjacent to each other. The bottom 16 of the crucible can also be made out of metal pipes. It can, however, also be made out of a fireproof material. At their ends, they are connected to vertical pipes for supplying coolant and/or for discharging coolant.
The heating is done by an induction coil (not shown) that surrounds the crucible wall 10, and via which high-frequency energy can be introduced into the contents of the crucible 8.
A skull crucible of this type has been made known, for example, from the patent EP 0 528 025 B1.
A skull crucible 8 operates as follows: the crucible 8 is filled with a glass batch or refuse glass (shards) or a mixture of them. The glass, and/or the melt, must be preheated at first in order to reach a certain minimum conductibility. The pre-heating is frequently done by burner heating. Once the coupling temperature has been reached, then the additional supply of energy via the irradiation of high-frequency energy can occur. Also, during the operation, it can be advantageous in special cases to heat the melt by burners which act on the melt from above, or by hot exhaust gases, in addition to the heating by high-frequency energy. This is especially necessary in some cases when using a skull crucible for refining. If the surface layer is namely cold and has a correspondingly higher viscosity, then bubbles are prevented from emerging out of the melt, or foam can form.
The skull principle can also be applied for containers other than crucibles. See the patent DE 199 39 782 A1, for example. In this process, several pipes are designed in a U-shape and lie adjacent to each other, so that they form a cage-like skull channel with each other that is open to the top. An induction coil then surrounds this channel in such a way that winding sections extend along the side walls of the channel.
The invention thus involves any type of device in which a container is made from many metal pipes, which themselves can be connected to a cooling medium, and in which high-frequency energy is introduced into the contents of the container.
The metal pipes mentioned are generally made of copper. This results in the following problems:
On the one hand, there is the danger that copper from the copper pipes gets into the melt. This leads to pronounced color streaking as a result of the copper impurity.
An additional problem consists in that gaseous or solid components escaping from the glass, such as, for example: HF, P2O5, B2O3, SO2, SO3, Cl2 precipitate onto the cooled part of the crucible not covered by the melt and cause corrosion there. This leads to a damage of the crucible and to impurity of the melt.
Furthermore, a removal of the residual glass when cleaning the container is expensive and painstaking, since this residual glass persistently adheres to the surfaces of the metal pipes.
The purpose of the invention is to create a device of the type named at the beginning in such a way that impurities of the glass melt due to the material of the metal pipe do not occur, that no corrosion occurs, and that the problem of the adherence of the glass melt does not occur.
The invention, in one form thereof, is a device for melting or refining inorganic substances, especially glass, wherein a plurality of metal pipes are connected to a cooling medium and lie next to each other in such a way that they form a container, and a high frequency coil is used for introducing energy into the contents of the container. The metal pipes are coated with a plastic having a decomposition temperature that is below the temperature of the melt and the cooling system is designed and arranged in such a way that the temperature of the boundary layer of the melt contacting the coated pipes is below the decomposition temperature of the coating material.
To achieve the intended purpose, different methods had been proposed previously. The use of platinum instead of copper is not only considerably more expensive, but also is not a perfect solution in terms of preventing impurities in the glass melt. The concept of using plastics had to be considered to be wrong from the beginning on, because of the low melting point of these materials. The inventors recognized, however, that plastics, in spite of their low softening temperature and in spite of their low decomposition temperature, come into consideration very well as a coating material, and they are suitable for solving the problems. As has been revealed in experiments, the cooling of the metal pipes caused the contact temperature between, on the one hand, the plastic of the plastic coating according to the invention, and on the other hand, the glass melt, to stay below the decomposition temperature of the plastic. The plastic coating remained undamaged after the experiments. The melting container could be reused.
By the plastic coating according to the invention, the most important of the partial purposes named above is achieved. Namely, a contamination of the melt does not occur. In the practical case, alkali zinc silicate glasses were melted in the skull crucible 8. Fibers, which were manufactured out of this glass, had an extremely low damping, which means that the coating prevented copper from diffusing out of the skull crucible into the melt. In a control experiment, uncoated copper pipes were used. These pipes led to considerable color streaking by copper impurity.
In another experiment, phosphate and fluorophosphate glasses were melted in a crucible designed according to the invention. The glasses exhibited a high optical purity, which otherwise is only obtained in platinum melting crucibles. Furthermore, the melt was free of platinum particles, which can, of course, not be guaranteed for platinum crucibles. The glasses were suitable for high-energy laser applications. Devices with plastic coating according to the invention are also suitable for oxide salt or metal melts. In the process, the expert will match the parameters of cooling, on the one hand, and the plastic properties, on the other hand, to each other.
The other important partial purpose of avoiding corrosion is achieved in a perfect manner. Corrosion does not occur in cooling pipes coated according to the invention.
Also, in the application of the coating according to the invention, no adhesion of glass residues occurs. Thus, the expense for the removal of these residues can be eliminated so that the cleaning of the crucible at the end of a melting or refining process is not a problem.